The present invention relates to the translation of frequencies within the Nyquist sampling band, and particularly but not exclusively to the translation of frequencies in a receiver or transmitter of a communication system.
The emergence of software defined radio, where the transmitter and receiver are implemented digitally, as a feasible alternative to the conventional Armstrong super-heterodyne approach has seen the migration of more and more of the receiver""s architecture to the discrete digital domain.
In a multi-carrier communication system a filter is required for each carrier frequency used by the system. Each filter has a centre frequency located in the Nyquist band, which is the portion of the sampled frequency spectrum located below half the sampling frequency. As the number of carrier frequencies, or channels, required to be supported by a communication system increases, the number of filters required to have different centre frequencies in the Nyquist band increases. Obviously, the frequency spectrum in the Nyquist band is fixed for a given sampling rate. Therefore to ensure correct operation of the communication system as the number of carrier frequencies used increases the design of the filters used must have a very precise bandwidth with accurately defined lower and upper cut-off frequencies.
The trend towards implementation of the transmitter and the receiver in the digital domain, coupled with ever-increasing bandwidths of interest, results for demands for increased flexibility in circuitry which provides the analogue to digital and digital to analogue domain conversion.
The cost of the circuitry thus increases owing to the need to provide a number of highly tuned filters at different centre frequencies. It is therefore desirable to provide means that minimise the circuitry necessary in implementing the filters.
It is therefore an object of the present invention to provide a signal processing means which results in the provision of an efficient filtering circuitry.
According to the present invention there is provided a method of processing a signal, comprising sampling the signal at a sampling frequency thereby generating discrete time components at the sampling interval; and inverting alternate ones of the discrete time components thereby translating a frequency component of the sampled signal in one part of the Nyquist band to another part of the Nyquist band.
The step of inverting alternate ones of the discrete time components may comprise multiplying the sampled signal by a reference signal at half the sampling frequency and having discrete time components at the sampling interval.
A frequency component in the upper half of the Nyquist band may be translated to a lower half of the Nyquist band or a frequency component in the lower half of the Nqyquist band may be translated to the upper half of the Nyquist band
The method may further comprise the step of selectively enabling the inverting step. The inverting step may be selectively enabled responsive to the frequency of the sampled signal being in a certain range. Each of the plurality of signals may have a frequency within the Nyquist band, and the step of inverting may be carried out selectively for the plurality of sampled signals, whereby the frequency components of selected ones of the signals are translated to another part of the Nyquist band.
Those signals having frequencies in the upper half of the Nyquist band may be translated to the lower half of the Nyquist band or those signals having frequencies in the lower half of the Nyquist band may be translated to the upper half of the Nyquist band.
The invention also provides a signal processing circuit comprising an input circuit connected to receive a signal; a sampling circuit connected to the input circuit and for sampling the signal at a sampling frequency to thereby generate discrete time components at the sampling interval; and inverting means connected to receive the sampled output and for inverting alternate ones of the discrete time components, whereby a frequency component of the sampled signal in one part of the Nyquist band is translated to another part of the Nyquist band.
The means for inverting may comprise a multiplier connected to multiply the sampled signal by a reference signal at half the sampling frequency and having discrete time components at the sampling interval.
A frequency component in the upper half of the Nyquist band may be translated to a lower half of the Nyquist band and a frequency component in the lower half of the Nqyquist band may be translated to the upper half of the Nyquist band
The signal processing circuit may further comprise a selection circuit for selectively enabling the means for inverting. The means for inverting may be selectively enabled responsive to the frequency of the sampled signal being in a certain range.
The input circuit may be connected to receive a plurality of signals having different frequencies, each of the plurality of signals having a frequency within the Nyquist band, wherein the inverting means is enabled selectively such that frequency components of selected ones of the signals are translated to another part of the Nyquist band.
Those signals having frequencies in the upper half of the Nyquist band may be translated to the lower half of the Nyquist band or those signals having frequencies in the lower half of the Nyquist band may be translated to the upper half of the Nyquist band.